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Proteases vs. antiproteases: The battle over milk digestion

Written by: Daniela Barile, Ph.D., David Dallas, Ph.D. | Issue # 7 | 2012

  • Milk partially digests its own proteins to release bioactive peptide fragments.
  • Milk peptides have health-enhancing functions ranging from immune-building to antimicrobial to nutritional.
  • Supplementation of diets with milk peptides may improve the health of premature infants and other immuno-compromised populations.

Surprisingly, milk protein digestion begins in the mammary gland, long before the milk is consumed. There, a battle rages between enzymes called proteases, which break down proteins, and antiproteases that act as shields by protecting other proteins from being completely digested. The result of this competition is a delicate balance of intact protein and partially digested protein segments. Research by Ferranti et al. revealed that milk, prior to ingestion, contained over 100 unique milk protein fragments (mostly deriving from the protein family called caseins) [3]. This finding demonstrates that proteases are busy breaking down proteins in the mammary gland.

So what does all of this mean? And what happens to these proteins after milk consumption if they’re already partially digested?

In vitro studies trying to replicate milk protein digestion show that milk peptides exert an array of effects, including behavioral, gastrointestinal, hormonal, immunological, neurological, and nutritional [2]. For example, fragments of milk casein aid in absorption of lipids and micronutrients, immune defense, antimicrobial protection, nerve transmission, social behavior modulation, analgesia (pain killers), decreasing diarrheal effects, and stimulating hormonal secretions.

However, all of these functions have only been shown in cell and animal models, and therefore cannot necessarily be extended to humans. Moreover, the peptide fragments tested so far have been artificially created with in vitro digestions of human milk proteins, and these fragments are different from those naturally occurring or created during infant digestion. Even the best scientists with advanced technologies cannot hope to accurately replicate the digestion of proteins—the gastrointestinal system is a far too complex environment.

So, though we know milk peptides have a large potential to be bioactive, we are still left with these questions: What peptides are actually produced and where are they along the digestive tract? How do these peptides actually affect human health?

Of course, in the gastrointestinal tract, milk proteins are exposed to even more enzymes that produce even more peptide fragments. There is little data on the products of protein digestion in infants. However, it is clear that not all proteins are digested into single amino acids. A small but significant percentage of some milk proteins, including lactoferrin and immunoglobulins, have even been found still intact in infant feces [4]. The annotation of peptides at various sites in digestion is critical for predicting their bioactivities. These fragments are so small and low in abundance, however, that even though their presence was known for decades, their identities could not be determined until recently. Fortunately, advances in mass spectrometry and database search now allow scientists to fully annotate large arrays of liberated peptides for their sequences and precursor proteins during digestion and to predict their function.

But that’s not the end of the story!

It has been known for quite some time that a variety of common commensal bacteria in the digestive tract can also help break down dietary protein [5]. Interestingly, even Bifidobacteria infantis, which is associated a healthy gut flora and which consumes human milk oligosaccharides, has also been shown to grow well on pre-digested milk lactoferrin and immunoglobulins [6]. However, until now, it was impossible to determine the fragments as they are released. The last obstacle to full characterization of the microbes’ effects is the difficulty in collecting the appropriate samples without invasive procedures.

The next step will be to take the peptides shown to exist in intact milk and at various places in the gastrointestinal tract and assay them for an array of bioactivities. Researchers at UC Davis are already exploring many exciting new bioactivities of these peptides.

This work is of critical importance because each year, more than half a million babies (about 1 in 8 deliveries) are born prematurely in the United States. Premature infants are incapable of digesting proteins as well as full term infants. That means premature infants are missing out on an army of protective bioactive peptides. This fact could be partially responsible for their poor health outcomes. Unfortunately for these infants, current neonatal infant care units focus principally on devices to monitor disease progression and little on improving infant nourishment. The development of a formula using an enzymatic pre-digestion of milk could provide exactly the right bioactive peptides from day one to ensure proper bacterial colonization of the premature gut for disease prevention. Besides improving the health of premature infants, bioactive milk peptides could potentially be isolated from bovine milk and employed to enhance health in specific consumer populations, ultimately enabling milk to be even more protective and supportive of human health.

References

1. Dallas DC, Underwood MA, Zivkovic AM, German JB. (2012) Digestion of Protein in Premature and Term Infants. J Nutr Disorders Ther. 2: 2161-0509.

2. Clare DA & Swaisgood HE. (2000) Bioactive milk peptides: a prospectus. J Dairy Sci. 83: 1187-1195.

3. Ferranti P, Traisci MV, Picariello G, Nasi A, Boschi V, Siervo M, Falconi C et al. (2004) Casein proteolysis in human milk: tracing the pattern of casein breakdown and the formation of potential bioactive peptides. J Dairy Res. 71: 74-87.

4. Davidson L. & Lönnerdal B. (1987) Persistence of human milk proteins in the breast-fed infant. Acta Paediatr. 76: 733-740.

5. Macfarlane G. & Allison C. (1986) Utilisation of protein by human gut bacteria. Fems Microbiology Letters. 38: 19-24.

6. Liepke C, Adermann K, Raida M, Magert HJ, Forssmann WG, Zucht HD. (2002) Human milk provides peptides highly stimulating the growth of bifidobacteria. Eur J of Biochem. 269: 712-718.

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